Although newspapers, magazines, office automation wastepapers, etc. have been recycled from of old, effective utilization of wastepapers has particularly recently become increasingly important in connection with global environmental problems such as protection of forest resources and refuse disposal. In addition, the use of deinked pulp has advanced to the utilization thereof as a higher-rank pulp, as exemplified by the use of deinked waste newspaper pulp for making printing paper B glade. Meanwhile, recent wastepapers have been getting into more and more severe circumstances from the standpoint of deinking in keeping with changes in printing techniques, printing methods, printing ink components, etc. Thus, it has been desired to develop a deinking process comprising a proper yield, a high whiteness and little residual ink. Accordingly, for example, improvements in a variety of chemicals have hitherto been made.
Meanwhile, as a decrease in the amount of treatment water for use in a deinking process is also desirable in aspects of the source of supply and environmental protection, white water is circulated and utilized, in general. White water refers to treatment water subjected to drainage and filtration in various steps of the deinking process. Specifically, in the flotation step, white water subjected to drainage in the washing or papermaking step after the flotation step is usually circulated and used as diluent water or the like before flotation for pulp slurry after the aging or bleaching step. Since, however, such circulation of white water gives rise to accumulation of ink fines and fibers in white water, the whiteness of deinked pulp is lowered as the circulation of white water is repeated. Further, used chemicals such as the deinking agent in particular are also concentrated to extremely increase foamability in the flotation step, thus causing a great decrease in the yield. The concentration of inorganic salts mainly comprising calcium ions and the like derived from a filler and the like is also not negligible as a cause of increasing foamability. In any event, the circulation of white water lowers the performance of deinked pulp and the efficiency of deinking. Despite the foregoing circumstances, there have surprisingly been made scarcely any investigations on the deinking agent and the deinking system that can maintain a deinking performance even in a white water circulation or recycling system. Thus, the fact is that a suitable amount of fresh water is used for dilution, or that additional waste water treatment equipment is operated.
JP-A 8-337085 discloses a deinking method in which floatation is conducted in the presence of a cationic compound, an amine compound, an acid salt of an amine compound or an amphoteric compound, but fails to use such a compound in floatation step using a white water.
Although circulation and reuse of white water in the deinking process are desirable in aspects of resources conservation, operation, etc., circulation of white water without causing decreases in the whiteness and yield has been getting into difficult circumstances. Accordingly, solution to this has been desired.